Enhancement of Thermal to Electrical Energy Conversion with Thermal Diodes
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Enhancement of Thermal to Electrical Energy Conversion with Thermal Diodes P. L. Hagelstein1 and Y. Kucherov2 1 Research Laboratory of Electronics, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 2 ENECO, Inc., 391-B Chipeta Way, Salt Lake City, UT 84108 ABSTRACT Experiments demonstrating thermal to electrical energy conversion using thermal diodes have shown an enhancement of the open circuit voltage over the thermoelectric open circuit voltage. Two different physical mechanisms are proposed to be responsible for the effects seen: (1) Thermionic injection from the emitter can occur when a temperature gradient is present, which induces an increased ohmic return current under zero-current conditions. (2) Blockage of the ohmic return current leads to a voltage increase for both thermoelectric and thermionic forward currents. Both effects increase the efficiency of energy conversion. Experiments show enhancements of the figure of merit in the range of 5-8 over the thermoelectric values. The best results are consistent with a single-side conversion efficiency in excess of 30% of the Carnot limit. OPEN CIRCUIT VOLTAGE ENHANCEMENT Thermionic behavior of semiconductors was described previously [1-4]. The first important observation we consider is the development of an enhancement of the open circuit voltage due to thermionic current injection in a thermal diode. Figure 1 shows the open circuit voltage as a function of temperature for two similar InSb thermal diodes. One thermal diode generates an open-circuit voltage, which is very close to the thermoelectric value. The other shows an open-circuit voltage that is enhanced by about a factor of two. Both diodes are 2 mm thick, with a gap doping of 7x1017 cm-3 Te. A thin emitter layer is present in both, with a doping of 3x1019 cm-3 Te and a thickness on the order of 2000 Å. The diode showing the enhanced voltage has in addition a thick (1 micron) emitter region with a doping of 2x1018 cm-3 Te. The voltage enhancement in this case is a factor of 2.2 at 200 oC. This voltage enhancement is in reasonable agreement with our estimates from a thermionic injection model for this device [5]. The resistance of these two samples is similar. For example, at 150 oC, the thermoelectric thermal diode has a resistance of 110 µΩ cm2, and the diode that produces an elevated open-circuit voltage has a resistance of 120 µΩ cm2 at the same temperature. An enhancement of the open circuit voltage increases the figure of merit Z associated with the thermal diode. For example, we might characterize the relevant thermoelectric properties of InSb through the use of a material-dependent figure of merit ZTE defined in the usual way
α 2σ (1) κ where σ is the electrical conductivity, α is the thermopower, and κ is the thermal conductivity. This material-dependent figure of merit would be appropriate for characterizing the performance expected from the InSb thermal diode described above that operates as a thermoelectric device. ZTE =
G8.37.1
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